Poly(meth)acrylate viscosity index improver, and lubricating oil composition and lubricating oil additive containing said viscosity index improver

ABSTRACT

The present invention provides a poly(meth)acrylate-based viscosity index improver comprising a polymer chain comprising a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), wherein Mw is 100000 or more, and Mw/Mn, is 1.6 or less. 
     
       
         
         
             
             
         
       
         
         
           
             [R 1  represents hydrogen or a methyl group, and R 2  represents a group represented by the following formula (3), and R 3  represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms. 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]

TECHNICAL FIELD

The present invention relates to a poly(meth)acrylate-based viscosity index improver, a lubricating oil additive and a lubricating oil composition containing the viscosity index improver.

BACKGROUND ART

Conventionally, in the field of lubricating oils, improvement of lubricating oils has been studied from the viewpoint of an energy saving property. Especially in recent years, a trend toward the global environmental protection has increased, and a need for an energy saving property improving effect for lubricating oils has been further strengthened.

For example, in the case of lubricating oils used for internal combustion engines such as a vehicle engine (also referred to as “lubricating oils for an internal combustion engine” or “engine oils”), as one means of improving a fuel saving property, a method of increasing a viscosity index of a lubricating oil by adding a viscosity index improver to a lubricating base oil has been known.

Moreover, for example, in the case of lubricating oils used for transmissions of vehicles, such as ATF, MTF, and CVTF (also referred to as “lubricating oils for a transmission” or “drive system oils”), as one means of improving a fuel saving property, there is a method of decreasing viscosity resistance by lowering the viscosity of a lubricating oil for a transmission. However, when the viscosity of a lubricating oil for a transmission is lowered, other problems such as oil leak and seizure may arise.

Therefore, as another method for improving a fuel saving property, there is a method involving use of a viscosity index improver. This method increases the viscosity index of a lubricating oil for a transmission by using a viscosity index improver, and suppresses the viscosity increase in a low-temperature region while maintaining the viscosity in a high-temperature region.

Regarding a viscosity index improver, the use of various viscosity index improvers has been proposed, and in particular, the use of poly(meth)acrylate-based viscosity index improvers has been often proposed (for example, refer to Patent Literatures 1 to 7).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. 7-48421

Patent Literature 2: Japanese Patent Application Laid-Open No. 7-62372

Patent Literature 3: Japanese Patent Application Laid-Open No. 6-145258

Patent Literature 4: Japanese Patent Application Laid-Open No. 3-100099

Patent Literature 5: Japanese Patent Application Laid-Open No. 2002-302687

Patent Literature 6: Japanese Patent Application Laid-Open No. 2004-124080

Patent Literature 7: Japanese Patent Application Laid-Open No. 2005-187736

SUMMARY OF INVENTION Technical Problem

However, for example, in lubricating oils for an internal combustion engine, in the case where the above-described conventional poly(meth)acrylate-based viscosity index improvers are used, there is a room for improvement in a high shear viscosity so as to achieve a practically sufficient fuel saving property. Especially, in 0W-20 whose requirement for fuel saving property is high, there is a need to maintain the high shear viscosity at a high level to some extent at 150° C., and on the other hand, to lower the high shear viscosity at 100° C. In contrast, in the conventional poly(meth)acrylate-based viscosity index improvers, it is difficult to lower the high shear viscosity at 100° C. while maintaining the high shear viscosity at 150° C.

Furthermore, recently, ensuring of fluidity at low temperature (in particular, extremely-low temperature) is needed in addition to a fuel saving property. However, the conventional poly(meth)acrylate-based viscosity index improvers are not necessarily sufficient in terms of achieving both a fuel saving property and low-temperature fluidity.

Moreover, for example, for lubricating oils for a transmission, as one cause of deterioration in a fuel saving property, there is friction loss of a gear in a driving device during power transmission. Therefore, if a lubricating oil whose viscosity resistance is low in a high shear condition can be achieved, friction loss can be decreased and a fuel saving property can be further improved. However, the above-described conventional viscosity index improvers attempt to improve a viscosity property in a high-temperature region and a low-temperature region by improving the viscosity index, and they are not considered to be sufficient in terms of a friction loss decreasing effect.

Furthermore, recently, ensuring of low-temperature fluidity is needed in addition to reduction of friction loss so as to be able to apply a lubricating oil over a wide range of temperature. However, the conventional viscosity index improvers are not sufficient in terms of achieving both a friction loss decreasing effect and low-temperature fluidity.

Therefore, an object of the present invention is to provide a viscosity index improver capable of achieving both a fuel saving property and low-temperature fluidity, a lubricating oil additive and a lubricating oil composition containing the viscosity index improver.

Moreover, another object of the present invention is to provide a viscosity index improver capable of sufficiently lowering a high shear viscosity at 100° C. and sufficiently ensuring low-temperature fluidity while maintaining a high shear viscosity at 150° C., a lubricating oil additive and a lubricating oil composition containing the viscosity index improver.

Furthermore, another object of the present invention is to provide a viscosity index improver which is capable of imparting a sufficient friction loss decreasing effect to a lubricating oil and ensuring low-temperature fluidity, a lubricating oil additive and a lubricating oil composition containing the viscosity index improver.

Solution to Problem

The present inventors made extensive research and found that a poly(meth)acrylate-based viscosity index improver which has a specific structural unit and in which the weight-average molecular weight Mw and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn satisfy specific conditions can sufficiently lower a high shear viscosity at 100° C. and ensure low-temperature fluidity while maintaining a high shear viscosity at 150° C., which leads to accomplish the present invention.

That is, the present invention provides a poly(meth)acrylate-based viscosity index improver comprising a polymer chain comprising a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), wherein the weight-average molecular weight Mw is 100000 or more, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, is 1.6 or less (hereinafter, referred to as “first poly(meth)acrylate-based viscosity index improver”).

[In the formulas (1) and (2), R¹ represents hydrogen or a methyl group, R² represents a group represented by the following formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

In the formula (3), m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]

Moreover, the present inventors made extensive research and found that a poly(meth)acrylate-based viscosity index improver which has a specific structural unit and in which the weight-average molecular weight and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, satisfy specific conditions can impart a friction decreasing effect and ensure low-temperature fluidity, which leads to accomplish the present invention.

That is, the present invention provides a poly(meth)acrylate-based viscosity index improver comprising a polymer chain comprising a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), wherein the weight-average molecular weight Mw is less than 100000, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, is 1.6 or less (hereinafter, referred to as “second poly(meth)acrylate-based viscosity index improver”).

[In the formulas (1) and (2), R¹ represents hydrogen or a methyl group, R² represents a group represented by the following formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

In the formula (3), m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]

Moreover, the present invention provides a lubricating oil additive comprising at least one selected from the above-described first poly(meth)acrylate-based viscosity index improver and second poly(meth)acrylate-based viscosity index improver.

Furthermore, the present invention provides a lubricating oil composition comprising a lubricating base oil, and at least one selected from the above-described first poly(meth)acrylate-based viscosity index improver and second poly(meth)acrylate-based viscosity index improver.

Advantageous Effects of Invention

According to the present invention, a viscosity index improver capable of achieving both a fuel saving property and low-temperature fluidity, a lubricating oil additive and a lubricating oil composition containing the viscosity index improver can be provided.

Moreover, according to the present invention, a viscosity index improver capable of sufficiently lowering a high shear viscosity at 100° C. and sufficiently ensuring low-temperature fluidity while maintaining a high shear viscosity at 150° C., a lubricating oil additive and a lubricating oil composition containing the viscosity index improver can be provided.

Furthermore, according to the present invention, a viscosity index improver which is capable of imparting a sufficient friction loss decreasing effect to a lubricating oil and ensuring of fluidity at low temperature, a lubricating oil additive and a lubricating oil composition containing the viscosity index improver can be provided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

First Embodiment First Poly(meth)acrylate-Based Viscosity Index Improver

A poly(meth)acrylate-based viscosity index improver according to the first embodiment comprises a polymer chain containing a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2). The weight-average molecular weight Mw (hereinafter, just referred to as “Mw” in some cases) of the poly(meth)acrylate-based viscosity index improver is 100000 or more, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn (hereinafter, just referred to as “Mn” in some cases), Mw/Mn (hereinafter, just referred to as “Mw/Mn” in some cases), is 1.6 or less.

[In the formulas (1) and (2), R¹ represents hydrogen or a methyl group, and R² represents a group represented by the following formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

In the formula (3), m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]

R¹ may be either hydrogen or a methyl group, and is preferably a methyl group.

From the viewpoint of lowering a viscosity, R² in which m is 5 to 16 and n is 4 to 15 is preferable, R² in which m is 6 to 15 and n is 6 to 10 is more preferable, and R² in which m is 7 to 10 and n is 6 to 9 is further preferable. In the case where two or more structural units represented by the above formula (1) are contained in the polymer chain, R¹s and R²s may be the same or different between the respective structural units.

As described above, the polymer chain contains the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2), and from the viewpoint of lowering a viscosity, contains preferably 20 to 80 mass %, more preferably 20 to 70 mass %, and further preferably 20 to 50 mass % of the structural unit represented by the above formula (1) based on the total amount of the structural units contained in the polymer chain. Moreover, from the viewpoint of a fuel saving property, the polymer chain contains preferably 20 to 80 mass %, more preferably 30 to 80 mass %, and further preferably 50 to 80 mass % of the structural unit represented by the above formula (2) based on the total amount of the structural units contained in the polymer chain. Furthermore, the polymer chain contains preferably 70 mass % or more, more preferably 80 mass % or more, further preferably 90 mass % or more, and most preferably 100 mass % of the sum of the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2) based on the total amount of the structural units contained in the polymer chain.

In the case where two or more structural units represented by the above formula (2) are contained in the polymer chain, R¹s and R³s may be the same or different between the respective structural units. In the case where two or more structural units in which R³s are different are contained, from the viewpoint of solubility of poly(meth)acrylate, the polymer chain contains preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 45 mass % of the structural unit in which R³ is a methyl group, based on the total amount of the structural units contained in the polymer chain. Moreover, from the viewpoint of low-temperature fluidity, the polymer chain contains preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 40 mass % of the structural unit in which R³ is a C18 alkyl group, based on the total amount of the structural units contained in the polymer chain.

The polymer chain may contain only the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2), or may further contain a structural unit other than these. Moreover, terminals of the polymer chain are not particularly limited. Among these polymer chains, a polymer chain containing only the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2), whose terminals are hydrogen atoms, that is, a polymer chain represented by the following formula (4) is preferable.

In the formula (4), R¹ represents hydrogen or a methyl group, R⁴ represents a group represented by the above formula (3), or a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms, and n represents an integer selected such that the Mw and the Mw/Mn satisfy the above-described conditions. For example, n is an integer of 400 to 2000.

The weight-average molecular weight Mw is 100000 or more, and it is preferably 125000 or more, more preferably 150000 or more, and further preferably 175000 or more from the viewpoint of a fuel saving performance. The upper limit of Mw is not particularly limited, and the Mw is, for example, 500000 or less.

The number average molecular weight Mn is arbitrarily selected such that the Mw/in satisfies the above-described condition. The Mn is preferably 75000 or more, more preferably 94000 or more, and further preferably 110000 or more from the viewpoint of a fuel saving performance. The upper limit of Mn is not particularly limited, and the Mn is, for example, 300000 or less.

The Mw/Mn is 1.6 or less, and it is preferably 1.5 or less, more preferably 1.4 or less, and further preferably 1.2 or less from the viewpoint of a fuel saving performance. Moreover, in the synthesis, the Mw/Mn is preferably 1.0 or more, more preferably 1.01 or more, and further preferably 1.02 or more.

It is to be noted that “the weight-average molecular weight Mw”, “the number average molecular weight Mn”, and “the ratio Mw/Mn of the weight-average molecular weight Mw to the number average molecular weight Mn” in the present invention mean Mw, Mn, and Mw/Mn (converted values with polystyrene (standard sample)) obtained by GPC analysis. Specifically, they are measured as follows, for example.

A solution whose sample concentration is 2 mass % is prepared by dilution using tetrahydrofuran as a solvent. The sample solution is analyzed using GPC equipment (Waters Alliance2695). The analysis is carried out at the flow rate of the solvent of 1 ml/min, by using a column whose analyzable molecular weight is 10000 to 256000, and a refractive index as a detector. It is to be noted that the relationship between the column retention time and the molecular weight is determined using a polystyrene standard whose molecular weight is clear and a calibration curve is separately made, and after that, the molecular weight is determined from the obtained retention time.

Although the manufacturing method of the poly(meth)acrylate-based viscosity index improver according to the present embodiment is not particularly limited, examples thereof include a method in which an initiator is added to a mixed solution containing an alkyl(meth)acrylate, a polymerization reagent, and a solvent to polymerize the alkyl(meth)acrylate at predetermined temperature.

As the alkyl(meth)acrylate, an alkyl(meth)acrylate represented by the following formula (5) and an alkyl(meth)acrylate represented by the following formula (6) can be used.

In the formulas (5) and (6), R¹ represents hydrogen or a methyl group, R² represents a group represented by the above formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

R¹ is preferably a methyl group. R² in which m is 5 to 16 and n is 4 to 15 is preferable, R² in which m is 6 to 15 and n is 6 to 10 is more preferable, and R² in which m is 7 to 10 and n is 6 to 9 is further preferable.

As the alkyl(meth)acrylate, as described above, the alkyl(meth)acrylate represented by the above formula (5) and the alkyl(meth)acrylate represented by the above formula (6) can be used, and the content of the alkyl(meth)acrylate represented by the above formula (5) is preferably 20 to 80 mass %, more preferably 20 to 70 mass %, and further preferably 20 to 50 mass % based on the total amount of the alkyl(meth)acrylate. Moreover, the content of the alkyl(meth)acrylate represented by the above formula (5) is preferably 20 to 80 mass %, more preferably 30 to 80 mass %, and further preferably 50 to 80 mass % based on the total amount of the alkyl(meth)acrylate.

As the alkyl(meth)acrylate represented by the above formula (6), one of the alkyl(meth)acrylate represented by the above formula (6) can be used alone, or two or more thereof can be mixed to be used, and preferably, two or more thereof are mixed to be used. In the case two or more thereof are mixed to be used, the content of methyl(meth)acrylate in which R² is a methyl group is preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 45 mass % based on the total amount of the alkyl(meth)acrylate. Moreover, the content of an alkyl(meth)acrylate in which R² is an alkyl group having 18 carbon atoms is preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 40 mass % based on the total amount of the alkyl(meth)acrylate.

As the polymerization reagent, for example, compounds having a thiocarbonyl group, such as cumyl dithiobenzoic acid, can be used. Examples of a preferred polymerization reagent include cumyl dithiobenzoic acid.

As the solvent, for example, highly-refined mineral oils, anisole, and toluene can be used. Examples of a preferred solvent include highly-refined mineral oils.

As the initiator, for example, azobisisobutyronitrile (AIBN), azobisdimethylvaleronitrile (AMBN), and azobismethylbutyronitrile (ADVN) can be used. Examples of a preferred initiator include azobisisobutyronitrile.

The reaction temperature when polymerizing the alkyl(meth)acrylate is preferably 70 to 120° C., more preferably 80 to 110° C., and further preferably 90 to 110° C. By making the reaction temperature be within the above-described range, the Mw/Mn of the obtained poly(meth)acrylate-based viscosity index improver becomes easy to be 1.6 or less. For example, when the reaction temperature is 90 to 110° C., the Mw/Mn tends to be 1.0 to 1.2, and when the reaction temperature is 80 to 110° C., the Mw/Mn tends to be 1.2 to 1.4, and when the reaction temperature is 70 to 120° C., the Mw/Mn tends to be 1.4 to 1.6.

The reaction time can be arbitrarily selected in accordance with the kinds and the amounts used of the alkyl(meth)acrylate, the polymerization reagent, the solvent, and the initiator, which are raw materials, reaction conditions such as a reaction temperature, and desired Mw and Mw/Mn of the poly(meth)acrylate. Examples of preferred reaction time include 10 to 14 hours.

The polymerization of the alkyl(meth)acrylate is preferably carried out in a nitrogen atmosphere.

Second Embodiment Lubricating Oil Additive

A lubricating oil additive according to the second embodiment of the present invention contains a poly(meth)acrylate-based viscosity index improver comprising a polymer chain containing a structural unit represented by the above formula (1) and a structural unit represented by the above formula (2), wherein the weight-average molecular weight Mw is 100000 or more, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, is 1.6 or less. It is to be noted that the poly(meth)acrylate-based viscosity index improver in the present embodiment is the same as the viscosity index improver in the above-described first embodiment, and an overlapping explanation is omitted here.

The lubricating oil additive may consists of only the above-described poly(meth)acrylate-based viscosity index improver, or may be a mixture of the viscosity index improver and other additives (that is, additive composition). In the case where the lubricating oil additive is a mixture of the viscosity index improver and other additives, the mixing ratio thereof is not particularly limited and can be arbitrarily selected depending on the intended use.

Examples of the other additives include additives such as viscosity index improvers other than the above-described poly(meth)acrylate-based viscosity index improver, antioxidants, antiwear agents (or extreme pressure agents), corrosion inhibitors, rust-preventive agents, viscosity index improvers, pour-point depressants, demulsifiers, metal deactivators, antifoamers, and ashless friction modifiers. One of these additives can be used alone, or two or more thereof can be used in combination.

Examples of the viscosity index improvers other than the above-described poly(meth)acrylate-based viscosity index improver include poly(meth)acrylate-based viscosity index improvers other than the above-described poly(meth)acrylate-based viscosity index improver, polyisobutene-based viscosity index improvers, ethylene-propylene copolymer-based viscosity index improvers, and styrene-butadiene hydrogenated copolymer-based viscosity index improvers.

Examples of the antioxidants include ashless antioxidants such as phenolic or amine antioxidants, and metallic antioxidants such as zinc, copper, or molybdenum antioxidants.

Examples of the phenolic antioxidants include 4,4′-methylenebis(2,6-di-tert-butylphenol), 4,4′-bis(2,6-di-tert-butylphenol), 4,4′-bis(2-methyl-6-tert-butylphenol), 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butylphenol), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol), 4,4′-isopropylidenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-nonyl phenol), 2,2′-isobutylidenebis(4,6-dimethylphenol), 2,2′-methylenebis(4-methyl-6-cyclohexylphenol), 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-α-dimethylamino-p-cresol, 2,6-di-tert-butyl-4-(N,N-dimethylaminomethylphenol), 4,4′-thiobis(2-methyl-6-tert-butylphenol), 4,4′-thiobis(3-methyl-6-tert-butylphenol), 2,2′-thiobis(4-methyl-6-tert-butylphenol), bis(3-methyl-4-hydroxy-5-tert-butylbenzyl) sulfide, bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide, 2,2′-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, stearyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and octyl-3-(3-methyl-5-di-tert-butyl-4-hydroxyphenyl)propionate. Two or more thereof may be mixed to be used.

Examples of the amine antioxidants include known amine antioxidants generally used for lubricating oils, such as aromatic amine compounds, alkyldiphenylamines, alkylnaphthylamines, phenyl-α-naphthylamine, and alkylphenyl-α-naphthylamines.

Examples of the corrosion inhibitors include benzotriazole, tolyltriazole, thiadiazole, and imidazole compounds.

Examples of the rust-preventive agents include petroleum sulfonates, alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenylsuccinic acid esters, and polyhydric alcohol esters.

Examples of the metal deactivators include imidazoline, pyrimidine derivatives, alkylthiadiazoles, mercaptobenzothiazole, benzotriazole or derivatives thereof, 1,3,4-thiadiazole polysulfide, 1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamate, 2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio)propionitrile.

Examples of the antifoamers include silicone oil whose kinematic viscosity at 25° C. is 1000 to 100000 mm²/s, alkenylsuccinic acid derivatives, esters of polyhydroxy aliphatic alcohols and long-chain fatty acids, methylsalicylate, and o-hydroxybenzyl alcohol.

As the ashless friction modifiers, arbitrary compounds generally used as ashless friction modifiers for lubricating oils can be used, and examples thereof include ashless friction modifiers such as amine compounds, fatty acid esters, fatty acid amides, fatty acids, aliphatic alcohols, and aliphatic ethers, each of which has at least one alkyl group or alkenyl group having 6 to 30 carbon atoms, in particular straight-chain alkyl group or straight-chain alkenyl group having 6 to 30 carbon atoms in a molecule. Moreover, nitrogen-containing compounds and acid-modified derivatives thereof and the like described in Japanese Patent Application Laid-Open No. 2009-286831 and various ashless friction modifiers exemplified in International Publication No. WO 2005/037967 Pamphlet can also be used.

Furthermore, the lubricating oil additive according to the present embodiment may further contain a solvent. As the solvent, highly-refined mineral oils, solvent-refined mineral oils, and synthetic oils can be used. Among them, it is preferable to use highly-refined mineral oils and solvent-refined mineral oils. In the case where the lubricating oil additive contains the solvent, the content of the solvent is preferably 5 to 75 mass %, and more preferably 30 to 60 mass % based on the total amount of the lubricating oil additive.

Third Embodiment Lubricating Oil Composition

A lubricating oil composition according to the third embodiment contains a lubricating base oil, and a poly(meth)acrylate-based viscosity index improver comprising a polymer chain containing a structural unit represented by the above formula (1) and a structural unit represented by the above formula (2), wherein the weight-average molecular weight Mw is 100000 or more, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, is 1.6 or less. The lubricating oil composition according to the present embodiment includes an aspect containing a lubricating base oil and the lubricating oil additive according to the above-described second embodiment. The poly(meth)acrylate-based viscosity index improver in the present embodiment is the same as the poly(meth)acrylate-based viscosity index improvers in the above-described first embodiment and second embodiment, and furthermore, other additives and a solvent which can be contained in the lubricating oil composition are the same as the other additives and the solvent in the second embodiment, and an overlapping explanation is omitted here.

The lubricating base oil is not particularly limited, and lubricating base oils used for general lubricating oils can be used. Specifically, mineral lubricating base oils, synthetic lubricating base oils, a mixture in which two or more lubricating base oils selected therefrom are mixed at an arbitrary ratio and the like can be used.

Examples of the mineral lubricating base oils include those obtained by refining a lubricating oil fraction obtained by reduced-pressure distillation of an atmospheric residue obtained by atmospheric distillation of a crude oil by carrying out one or more treatment, such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, and hydrorefining, and base oils manufactured by a method of isomerizing wax-isomerized mineral oils and GTL waxes (gas-to-liquid waxes).

Examples of the synthetic lubricating oils include polybutene or hydrides thereof; poly-α-olefins such as 1-octene oligomer and 1-decene oligomer, or hydrides thereof; diesters such as ditridecyl glutarate, di-2-ethylhexyl adipate, diisodecyl adipate, ditridecyl adipate, and di-2-ethylhexyl sebacate; polyol esters such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethylhexanoate, and pentaerythritol pelargonate; aromatic synthetic oils such as alkylnaphthalenes and alkylbenzenes, and mixtures thereof.

The kinematic viscosity at 100° C. of the lubricating base oil is preferably 2.5 to 10.0 mm²/s, more preferably 3.0 to 8.0 mm²/s, and further preferably 3.5 to 6.0 mm²/s. Moreover, the viscosity index of the lubricating base oil is preferably 90 to 165, more preferably 100 to 155, and further preferably 120 to 150.

The saturated component of the lubricating base oil by chromatography analysis is preferably 80% or more, more preferably 85% or more, further preferably 90% or more, and most preferably 95% or more so as to make it easy to exert an effect of additives such as the poly(meth)acrylate-based viscosity index improver according to the first embodiment.

The content of the poly(meth)acrylate-based viscosity index improver according to the first embodiment is preferably 0.1 to 20.0 mass %, more preferably 0.5 to 15.0 mass %, and further preferably 1.0 to 10.0 mass % based on the total amount of the lubricating oil composition. When the content is the above-described lower limit or more, a sufficient effect of addition becomes easy to be obtained, and on the other hand, when the content is the above-described upper limit or less, shear stability increases and fuel consumption sustainability is improved.

The kinematic viscosity at 100° C. of the lubricating oil composition is preferably 3.0 to 16.3 mm²/s, more preferably 3.5 to 12.5 mm²/s, and further preferably 4.0 to 9.3 mm²/s. When the kinematic viscosity at 100° C. is the above-described lower limit or more, a lubricating property becomes easy to be ensured, and on the other hand, when the kinematic viscosity at 100° C. is the above-described upper limit or less, a fuel saving property is further improved. It is to be noted that the kinematic viscosity at 100° C. in the present invention means a kinematic viscosity at 100° C. defined by JIS K-2283-1993.

The viscosity index of the lubricating oil composition is preferably 150 to 250, more preferably 160 to 240, and further preferably 170 to 230. When the viscosity index is the above-described lower limit or more, a fuel saving property can be further improved, and moreover, the low-temperature viscosity becomes easy to be lowered while maintaining the HTHS viscosity. On the other hand, when the viscosity index is the above-described upper limit or less, low-temperature fluidity, solubility of additives, and compatibility with a sealing material can be ensured. It is to be noted that the viscosity index in the present invention means a viscosity index defined by JIS K 2283-1993.

The HTHS viscosity at 150° C. of the lubricating oil composition is preferably 1.7 mPa·s or more, more preferably 2.0 mPa·s or more, further preferably 2.3 mPa·s or more, and most preferably 2.6 mPa·s or more. When the HTHS viscosity at 150° C. is the above-described lower limit or more, evaporation of the lubricating oil composition can be suppressed, and a lubricating property can be ensured. Moreover, the HTHS viscosity at 100° C. of the lubricating oil composition is preferably 5.2 mPa·s or less, more preferably 5.1 mPa·s or less, and further preferably 5.0 mPa·s or less. When the HTHS viscosity at 100° C. is the above-described upper limit or less, a higher fuel saving property can be obtained. It is to be noted that the HTHS viscosity at 150° C. or 100° C. in the present invention means a high temperature high shear viscosity at 150° C. or 100° C. defined by ASTM D-4683.

The MRV viscosity at −40° C. of the lubricating oil composition is preferably 60000 mPa·s or less, more preferably 40000 mPa·s or less, and further preferably 30000 mPa·s or less. When the MRV viscosity at −40° C. is the above-described upper limit or less, a pumping property is excellent at low temperature. It is to be noted that the MRV viscosity at −40° C. in the present invention means a MRV viscosity at −40° C. defined by ASTM D-4684.

The viscosity index improver according to the first embodiment, the lubricating oil additive according to the second embodiment, and the lubricating oil composition according to the third embodiment, which are described above, can be used in a wide range of fields such as lubricating oils for an internal combustion engine and drive system lubricating oils, and in particular, are useful in the field of lubricating oils for an internal combustion engine. Fuel of the internal combustion engine in this case may be either gasoline or diesel fuel.

Fourth Embodiment Second Poly(meth)acrylate-Based Viscosity Index Improver

A poly(meth)acrylate-based viscosity index improver according to the fourth embodiment comprises a polymer chain containing a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2). The weight-average molecular weight Mw (hereinafter, just referred to as “Mw” in some cases) of the poly(meth)acrylate-based viscosity index improver is less than 100000, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn (hereinafter, just referred to as “Mw” in some cases), Mw/Mn (hereinafter, just referred to as “Mw/Mn” in some cases), is 1.6 or less.

[In the formulas (1) and (2), R¹ represents hydrogen or a methyl group, R² represents a group represented by the following formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

In the formula (3), m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]

R¹ may be either hydrogen or a methyl group, and is preferably a methyl group.

As R², from the viewpoint of lowering a viscosity, one in which m is 5 to 16 and n is 4 to 15 is preferable, one in which m is 6 to 15 and n is 6 to 10 is more preferable, and one in which m is 7 to 10 and n is 6 to 9 is further preferable. In the case where the number of the structural units contained in the polymer chain and represented by the above formula (1) is 2 or more, R¹s and R²s may be the same or different between the respective structural units.

As described above, the polymer chain contains the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2), and from the viewpoint of lowering a viscosity of compounded oil, contains preferably 20 to 80 mass %, more preferably 20 to 70 mass %, and further preferably 20 to 50 mass % of the structural unit represented by the above formula (1) based on the total amount of the structural units contained in the polymer chain. Moreover, from the viewpoint of a fuel saving property, the polymer chain contains preferably 20 to 80 mass %, more preferably 20 to 70 mass %, and further preferably 50 to 80 mass % of the structural unit represented by the above formula (2) based on the total amount of the structural units contained in the polymer chain. Furthermore, the polymer chain contains preferably 70 mass % or more, more preferably 80 mass % or more, further preferably 90 mass % or more, and most preferably 100 mass % of the sum of the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2) based on the total amount of the structural units contained in the polymer chain.

In the case where two or more structural units represented by the above formula (2) are contained in the polymer chain, R¹s and R³s may be the same or different between the respective structural units. In the case where two or more structural units in which R³s are different are contained, from the viewpoint of solubility of poly(meth)acrylate, the polymer chain contains preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 45 mass % of the structural unit in which R³ is a methyl group, based on the total amount of the structural units contained in the polymer chain. Moreover, from the viewpoint of low-temperature fluidity, the polymer chain contains preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 40 mass % of the structural unit in which R³ is an alkyl group having 18 or more carbon atoms, based on the total amount of the structural units contained in the polymer chain.

The polymer chain may contain only the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2), or may further contain a structural unit other than these. Moreover, terminals of the polymer chain are not particularly limited. Among these polymer chains, a polymer chain containing only the structural unit represented by the above formula (1) and the structural unit represented by the above formula (2), whose terminals are hydrogen atoms, that is, a polymer chain represented by the following formula (4) is preferable.

In the formula (4), R¹ represents hydrogen or a methyl group, R⁴ represents a group represented by the above formula (3), or a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms, and n represents an integer selected such that the Mw and the Mw/Mn satisfy the above-described conditions. For example, n is an integer of 40 to 450.

The weight-average molecular weight Mw is less than 100000, and it is preferably 80000 or less, more preferably 70000 or less, and further preferably 60000 or less from the viewpoint of a fuel saving property. The lower limit of Mw is not particularly limited, and the Mw is, for example, 10000 or more.

The number average molecular weight Mn is arbitrarily selected such that the Mw/Mn satisfies the above-described condition. The Mn is preferably 6000 or more, more preferably 10000 or more, and further preferably 12500 or more from the viewpoint of a fuel saving property. The upper limit of Mn is not particularly limited, and the Mn is, for example, 60000 or less.

The Mw/Mn is 1.6 or less, and from the viewpoint of a fuel saving property, is preferably 1.5 or less, more preferably 1.4 or less, and further preferably 1.3 or less. Moreover, the Mw/Mn is, from the viewpoint of a fuel saving property, preferably 1.0 or more, more preferably 1.01 or more, and further preferably 1.02 or more.

It is to be noted that “the weight-average molecular weight Mw”, “the number average molecular weight Mn”, and “the ratio Mw/Mn of the weight-average molecular weight Mw to the number average molecular weight Mn” in the present invention mean Mw, Mn, and Mw/Mn (converted values with polystyrene (standard sample)) obtained by GPC analysis. Specifically, they are measured as follows, for example.

A solution whose sample concentration is 2 mass % is prepared by dilution using tetrahydrofuran as a solvent. The sample solution is analyzed using GPC equipment (Waters Alliance2695). The analysis is carried out at the flow rate of the solvent of 1 ml/min, by using a column whose analyzable molecular weight is 10000 to 256000, and a refractive index as a detector. It is to be noted that the relationship between the column retention time and the molecular weight is determined using a polystyrene standard whose molecular weight is definite and the molecular weight is determined from the obtained retention time based on the calibration curve which is separately made.

Although the manufacturing method of the poly(meth)acrylate-based viscosity index improver according to the present embodiment is not particularly limited, examples thereof include a method in which an initiator is added to a mixed solution containing an alkyl(meth)acrylate, a polymerization reagent, and a solvent to polymerize the alkyl(meth)acrylate at predetermined temperature.

As the alkyl(meth)acrylate, an alkyl(meth)acrylate represented by the following formula (5) and an alkyl(meth)acrylate represented by the following formula (6) can be used.

In the formulas (5) and (6), R¹ represents hydrogen or a methyl group, R² represents a group represented by the above formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

R¹ is preferably a methyl group. As R², one in which m is 5 to 16 and n is 4 to 15 is preferable, one in which m is 6 to 15 and n is 6 to 10 is more preferable, and one in which m is 7 to 10 and n is 6 to 9 is further preferable.

As the alkyl(meth)acrylate, as described above, the alkyl(meth)acrylate represented by the above formula (5) and the alkyl(meth)acrylate represented by the above formula (6) can be used, and the content of the alkyl(meth)acrylate represented by the above formula (5) is preferably 20 to 80 mass %, more preferably 20 to 70 mass %, and further preferably 20 to 50 mass % based on the total amount of the alkyl(meth)acrylate. Moreover, the content of the alkyl(meth)acrylate represented by the above formula (5) is preferably 20 to 80 mass %, more preferably 30 to 80 mass %, and further preferably 50 to 80 mass % based on the total amount of the alkyl(meth)acrylate.

As the alkyl(meth)acrylate represented by the above formula (6), one of the alkyl(meth)acrylate represented by the above formula (6) can be used alone, or two or more thereof can be mixed to be used, and preferably, two or more thereof are mixed to be used. In the case where two or more thereof are mixed to be used, the content of methyl(meth)acrylate in which R² is a methyl group is preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 45 mass % based on the total amount of the alkyl(meth)acrylate. Moreover, the content of an alkyl(meth)acrylate in which R² is a C18 alkyl group is preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and further preferably 20 to 40 mass % based on the total amount of the alkyl(meth)acrylate.

As the polymerization reagent, for example, compounds containing a thiocarbonyl group, such as cumyl dithiobenzoic acid, can be used. Examples of a preferred polymerization reagent include cumyl dithiobenzoic acid.

As the solvent, for example, highly-refined mineral oils, anisole, and toluene can be used. Examples of a preferred solvent include highly-refined mineral oils.

As the initiator, for example, azobisisobutyronitrile, azobisdimethylvaleronitrile, and azobismethylbutyronitrile can be used. Examples of a preferred initiator include azobisisobutyronitrile.

The reaction temperature when polymerizing the alkyl(meth)acrylate is preferably 70 to 120° C., more preferably 80 to 110° C., and further preferably 80 to 120° C. By making the reaction temperature be within the above-described range, Mw/Mn of the obtained poly(meth)acrylate-based viscosity index improver becomes easy to be 1.6 or less. For example, when the reaction temperature is 90 to 100° C., Mw/Mn tends to be 1.0 to 1.2, and when the reaction temperature is 100 to 110° C., Mw/Mn tends to be 1.2 to 1.4, and when the reaction temperature is 110 to 120° C., Mw/Mn tends to be 1.4 to 1.6.

The reaction time can be arbitrarily selected in accordance with the kinds and the amounts used of the alkyl(meth)acrylate, the polymerization reagent, the solvent, and the initiator, which are raw materials, reaction conditions such as a reaction temperature, and desired Mw and Mw/Mn of the poly(meth)acrylate. Examples of preferred reaction time include 10 to 14 hours.

The polymerization of the alkyl(meth)acrylate is preferably carried out in a nitrogen atmosphere.

Fifth Embodiment Lubricating Oil Additive

A lubricating oil additive according to the fifth embodiment of the present invention contains a poly(meth)acrylate-based viscosity index improver comprising a polymer chain containing a structural unit represented by the above formula (1) and a structural unit represented by the above formula (2), wherein the weight-average molecular weight Mw is less than 100000, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, is 1.6 or less. It is to be noted that the poly(meth)acrylate-based viscosity index improver in the present embodiment is the same as the viscosity index improver in the above-described fourth embodiment, and an overlapping explanation is omitted here.

The lubricating oil additive may be one composed of only the above-described poly(meth)acrylate-based viscosity index improver, or may be a mixture of the viscosity index improver and other additives (that is, additive composition). In the case where the lubricating oil additive is a mixture of the viscosity index improver and other additives, the mixing ratio thereof is not particularly limited and can be arbitrarily selected depending on the intended use. The other additives are the same as the other additives in the above-described second embodiment, and an overlapping explanation is omitted here.

Furthermore, the lubricating oil additive according to the present embodiment may further contain a solvent. As the solvent, highly-refined mineral oils, solvent-refined mineral oils, and synthetic oils can be used. Among them, highly-refined mineral oils are preferably used. In the case where the lubricating oil additive contains the solvent, the content of the solvent is preferably 5 to 75 mass %, and more preferably 30 to 60 mass % based on the total amount of the lubricating oil additive.

Sixth Embodiment Lubricating Oil Composition

A lubricating oil composition according to the sixth embodiment contains a lubricating base oil, and a poly(meth)acrylate-based viscosity index improver comprising a polymer chain containing a structural unit represented by the above formula (1) and a structural unit represented by the above formula (2), wherein the weight-average molecular weight Mw is less than 100000, and the ratio of the weight-average molecular weight Mw to the number average molecular weight Mn, Mw/Mn, is 1.6 or less. The lubricating oil composition according to the present embodiment includes an aspect containing a lubricating base oil and the lubricating oil additive according to the above-described fifth embodiment. The poly(meth)acrylate-based viscosity index improver in the present embodiment is the same as the poly(meth)acrylate-based viscosity index improvers in the above-described fourth embodiment and fifth embodiment, and furthermore, other additives and a solvent which can be contained in the lubricating oil composition are the same as the other additives and the solvent in the second embodiment, and an overlapping explanation is omitted here.

The lubricating base oil is the same as the lubricating base oil in the above-described third embodiment, and an overlapping explanation is omitted here.

The content of the poly(meth)acrylate-based viscosity index improver according to the fourth embodiment is preferably 0.1 to 20.0 mass %, more preferably 0.5 to 15.0 mass %, and further preferably 1.0 to 10.0 mass % based on the total amount of the lubricating oil composition. When the content is the above-described lower limit or more, a sufficient effect of addition becomes easy to be obtained, and on the other hand, when the content is the above-described upper limit or less, shear stability increases and fuel consumption sustainability is improved.

The kinematic viscosity at 100° C. of the lubricating oil composition is preferably 2.0 to 16.3 mm²/s, more preferably 2.5 to 12.5 mm²/s, and further preferably 3.0 to 10.0 mm²/s. When the kinematic viscosity at 100° C. is the above-described lower limit or more, a lubricating property becomes easy to be ensured, and on the other hand, when the kinematic viscosity at 100° C. is the above-described upper limit or less, a fuel saving property is further improved. It is to be noted that the kinematic viscosity at 100° C. in the present invention means a kinematic viscosity at 100° C. defined by JIS K-2283-1993.

The viscosity index of the lubricating oil composition is preferably 130 to 250, more preferably 140 to 240, and further preferably 160 to 230. When the viscosity index is the above-described lower limit or more, a fuel saving property can be further improved, and moreover, the low-temperature viscosity becomes easy to be lowered while maintaining the HTHS viscosity. On the other hand, when the viscosity index is the above-described upper limit or less, low-temperature fluidity, solubility of additives, and compatibility with a sealing material can be ensured. It is to be noted that the viscosity index in the present invention means a viscosity index defined by JIS K 2283-1993.

The BF viscosity at −40° C. of the lubricating oil composition is preferably 20000 mPa·s or less, more preferably 18000 mPa·s or less, and further preferably 16000 mPa·s or less. When the BF viscosity at −40° C. is the above-described upper limit or less, low-temperature fluidity is excellent and it becomes easy for a lubricating oil to flow at low temperature. It is to be noted that the BF viscosity at −40° C. in the present invention means a BF viscosity at −40° C. defined by JPI-5S-26-99.

The viscosity index improver according to the fourth embodiment, the lubricating oil additive according to the fifth embodiment, and the lubricating oil composition according to the sixth embodiment, which are described above, can be used in a wide range of fields such as lubricating oils for an internal combustion engine and drive system lubricating oils, and in particular, are useful in the field of drive system lubricating oils. A driving device in this case may be any of an automatic transmission (AT), a continuously variable transmission (CVT), and a stepped transmission (TM).

EXAMPLES

Hereinafter, the present invention will be described in further detail with reference to Examples, but the present invention is not limited to the following Examples.

Example 1-1

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 1-1”).

12 g of methyl methacrylate (compound in which both R¹ and R³ in the formula (6) are methyl groups, and hereinafter, designated as “C1-MA”), 9 g of 2-octyldodecyl methacrylate (compound in which R¹ and R² in the formula (5) are a methyl group and the formula (3) having m=9 and n=6, respectively, and hereinafter, designated as “A2”), 9 g of stearyl methacrylate (compound in which R¹ and R³ in the formula (6) are a methyl group and a stearyl group (straight-chain alkyl group having 18 carbon atoms), respectively, and hereinafter, designated as “C18-MA”), 0.031 g of cumyl dithiobenzoic acid (CDTBA), and 30 g of a highly-refined mineral oil as a solvent were charged into a 300 ml five-neck separable flask fitted with an anchor-type metal stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a sample inlet, and a homogeneous solution was obtained under stirring. The solution was cooled to 0° C. on an ice bath, and vacuum deaeration/nitrogen purge of a reaction system was carried out 5 times using a diaphragm pump. Furthermore, from the sample inlet, as a radical initiator, 0.005 g of azobisisobutyronitrile (AIBN) was charged under nitrogen flow, and then, polymerization was carried out for 12 hours at the solution temperature of 110° C. under a nitrogen atmosphere to obtain a solution containing a poly(meth)acrylate-based viscosity index improver.

For the obtained poly(meth)acrylate-based viscosity index improver, the weight-average molecular weight Mw and the number average molecular weight Mn were measured by GPC analysis. As a result, the weight-average molecular weight Mw was 233000, the number average molecular weight Mn was 150000, and the Mw/Mn was 1.55. The procedure of the GPC analysis is as follows.

A solution whose sample concentration is 2 mass % was prepared by dilution using tetrahydrofuran as a solvent. The sample solution was analyzed using GPC equipment (Waters Alliance2695). The analysis was carried out at the flow rate of the solvent of 1 ml/min, by using a column whose analyzable molecular weight is 10000 to 256000, and a refractive index as a detector. It is to be noted that the relationship between the column retention time and the molecular weight was determined using a polystyrene standard whose molecular weight is definite and the molecular weight was determined from the obtained retention time based on the calibration curve which was separately made.

Example 1-2

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 1-2”).

12 g of methyl methacrylate (C1-MA), 9 g of 2-octyldodecyl methacrylate (A2), 9 g of stearyl methacrylate (C18-MA), 0.030 g of cumyl dithiobenzoic acid (CDTBA), and 30 g of a highly-refined mineral oil as a solvent were charged into a 300 ml five-neck separable flask fitted with an anchor-type metal stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a sample inlet, and a homogeneous solution was obtained under stirring. The solution was cooled to 0° C. on an ice bath, and vacuum deaeration/nitrogen purge of a reaction system was carried out 5 times using a diaphragm pump. Furthermore, from the sample inlet, as a radical initiator, 0.005 g of azobisisobutyronitrile (AIBN) was charged under nitrogen flow, and then, polymerization was carried out for 12 hours at the solution temperature of 100° C. under a nitrogen atmosphere to obtain a solution containing a poly(meth)acrylate-based viscosity index improver.

For the obtained poly(meth)acrylate-based viscosity index improver, GPC analysis was carried out in the same manner as Example 1-1, and as a result, the weight-average molecular weight Mw was 228000, the number average molecular weight Mn was 171000, and the Mw/Mn was 1.33.

Example 1-3

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 1-3”).

12 g of methyl methacrylate (C1-MA), 9 g of 2-octyldodecyl methacrylate (A2), 9 g of stearyl methacrylate (C18-MA), 0.032 g of cumyl dithiobenzoic acid (CDTBA), and 30 g of a highly-refined mineral oil as a solvent were charged into a 300 ml five-neck separable flask fitted with an anchor-type metal stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a sample inlet, and a homogeneous solution was obtained under stirring. The solution was cooled to 0° C. on an ice bath, and vacuum deaeration/nitrogen purge of a reaction system was carried out 5 times using a diaphragm pump. Furthermore, from the sample inlet, as a radical initiator, 0.005 g of azobisisobutyronitrile (AIBN) was charged under nitrogen flow, and then, polymerization was carried out for 12 hours at the solution temperature of 90° C. under a nitrogen atmosphere to obtain a solution containing a poly(meth)acrylate-based viscosity index improver.

For the obtained poly(meth)acrylate-based viscosity index improver, GPC analysis was carried out in the same manner as Example 1-1, and as a result, the weight-average molecular weight Mw was 210000, the number average molecular weight Mn was 194000, and the Mw/Mn was 1.08.

Comparative Example 1-3

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 1-4”).

30 g of a highly-refined mineral oil as a solvent was charged into a 300 ml four-neck reaction flask fitted with a stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a dropping funnel for introducing a sample, and it was stirred for 1 hour in an oil bath at 85° C. while carrying out nitrogen purge. A raw material in which 12 g of methyl methacrylate (C1-MA), 9 g of 2-octyldodecyl methacrylate (A2), and 9 g of stearyl methacrylate (C18-MA) as raw material monomers, and 0.091 g of azobisisobutyronitrile (AIBN) as a radical initiator are mixed was charged into the dropping funnel for introducing a sample, and the raw material was dropped in the reaction flask for 70 minutes. After that, polymerization was carried out for 8 hours at 85° C. under nitrogen flow while maintaining stirring to obtain a solution containing a poly(meth)acrylate-based viscosity index improver. After that, unreacted monomers were removed from the above-described solution by carrying out vacuum distillation for 3 hours at 130° C. and 1 mmHg.

For the obtained poly(meth)acrylate-based viscosity index improver, GPC analysis was carried out in the same manner as Example 1-1, and as a result, the weight-average molecular weight Mw was 88000, the number average molecular weight Mn was 72000, and the Mw/Mn was 1.22.

Examples 1-4 to 1-23, Comparative Examples 1-1 to 1-2, 1-4 to 1-7, Reference Example 1-1

A poly(meth)acrylate-based viscosity index improver was synthesized in the same manner as any of the above-described Synthesis Conditions 1-1 to 1-4 other than changing the amount of the raw material blended as shown in Tables 1, 3, 5, 7, 9, and 11. It is to be noted that, in Tables, C12-MA represents a compound in which R¹ and R² in the formula (6) are a methyl group and a dodecyl group (straight-chain alkyl group having 12 carbon atoms), respectively, and moreover, A1: m=7, n=6 or the like represents a compound in which R¹ and R² in the formula (5) are a methyl group and the formula (3) having m=7 and n=6, respectively or the like. Mw, Mn, and Mw/Mn of the obtained poly(meth)acrylate-based viscosity index improver are shown in Tables 2, 4, 6, 8, 10, and 12.

<Preparation of Lubricating Oil Composition>

The poly(meth)acrylate-based viscosity index improver obtained in each of Examples 1-1 to 1-23, Comparative Examples 1-1 to 1-7, and Reference Example 1-1, performance additives including a metallic (calcium sulfonate) cleaner, an ashless dispersant (succinimide), a friction modifier (glycerin monooleate), and a wear inhibitor (zinc dithiophosphate), and a highly-refined mineral oil (Group III base oil, kinematic viscosity at 100° C.: 4.2 mm²/s, VI: 125) were blended at a ratio shown in Tables 2, 4, 6, 8, 10, and 12 to prepare a lubricating oil composition.

<Evaluation of Lubricating Oil Composition>

For each lubricating oil composition of Examples 1-1 to 1-23, Comparative Examples 1-1 to 1-7, and Reference Example 1-1, the kinematic viscosity at 100° C., the viscosity index, the HTHS viscosities at 100° C. and 150° C., and the MRV viscosities at −40° C. were respectively measured by methods in conformity with the following. The results are shown in Tables 2, 4, 6, 8, 10, and 12. It is to be noted that, in Tables, “Y.S.” in the item of the MRV viscosity represents yield stress and means out of standard.

kinematic viscosity: JIS K-2283-1993

viscosity index: JIS K-2283-1993

HTHS viscosity: ASTM D-4683

MRV viscosity: ASTM D-4684

TABLE 1 Example Example Example Example Example Example 1-1 1-2 1-3 1-4 1-5 1-6 Amount Blended (g) C1-MA 12.0  12.0  12.0  12.0  12.0  12.0  A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 9.0 9.0 9.0 9.0 9.0 9.0 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 9.0 9.0 9.0 9.0 9.0 9.0 C12-MA — — — — — — CDTBA  0.031  0.030  0.032  0.079  0.081  0.082 AIBN  0.005  0.005  0.005  0.013  0.013  0.014 Synthesis Condition 1-1 1-2 1-3 1-1 1-2 1-3 Yield (%) 94.8  93.6  94.5  95.1  95.2  94.2 

TABLE 2 Example Example Example Example Example Example 1-1 1-2 1-3 1-4 1-5 1-6 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 40 40 A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 30 30 30 30 30 30 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 30 30 30 30 30 30 C12-MA — — — — — — Mw 233,000 228,000 210,000 107,000 108,000 103,000 Mn 150,000 171,000 194,000 70.000 82,000 92,000 Mw/Mn 1.55 1.33 1.08 1.53 1.31 1.11 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Balance Performance Additive 9.5 9.5 9.5 9.5 9.5 9.5 Viscosity Index Improver 2.5 2.8 2.6 2.4 2.4 2.5 Kinematic Viscosity 7.21 7.33 7.38 7.28 7.45 7.18 (mm²/s)/100° C. Viscosity Index 188 190 191 190 185 193 HTHS Viscosity (mPa · s) 150° C. 2.6 2.6 2.6 2.6 2.6 2.6 100° C. 4.87 4.83 4.72 4.96 4.91 4.90 MRV Viscosity 13,500 12,000 11,300 14,200 14,100 14,600 (mPa · s)/−40° C.

TABLE 3 Example Example Example Example Example Example 1-7 1-8 1-9 1-10 1-11 1-12 Amount Blended (g) C1-MA  6.0 7.5 9.0 13.5  12.0 12.0 A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 12.0 15.0  9.0 9.0  6.0 12.0 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 12.0 7.5 12.0  7.5 12.0  6.0 C12-MA — — — — — — CDTBA   0.031  0.032  0.029  0.022   0.028   0.021 AIBN   0.005  0.005  0.005  0.004   0.004   0.004 Synthesis Condition 1-3 1-3 1-3 1-3 1-3 1-3 Yield (%) 94.8 93.6  94.5  95.1  95.2 94.2

TABLE 4 Example Example Example Example Example Example 1-7 1-8 1-9 1-10 1-11 1-12 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 20 25 30 45 40 40 A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 40 50 30 30 20 40 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 40 25 40 25 40 20 C12-MA — — — — — — Mw 233,000 228,000 210,000 267,000 258,000 273,000 Mn 197,000 202,000 183,000 245,000 217,000 241,000 Mw/Mn 1.18 1.13 1.15 1.09 1.19 1.13 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Balance Performance Additive 9.5 9.5 9.5 9.5 9.5 9.5 Viscosity Index Improver 2.5 2.8 2.6 2.4 2.4 2.5 Kinematic Viscosity 7.21 7.33 7.38 7.28 7.45 7.18 (mm²/s)/100° C. Viscosity Index 188 190 191 190 185 193 HTHS Viscosity (mPa · s) 150° C. 2.6 2.6 2.6 2.6 2.6 2.6 100° C. 4.97 4.95 4.93 4.81 4.91 4.81 MRV Viscosity 14,800 13,000 14,300 12,200 14,300 12,600 (mPa · s)/−40° C.

TABLE 5 Example Example Example Example Example Example 1-13 1-14 1-15 1-16 1-17 1-18 Amount Blended (g) C1-MA 12.0  12.0  12.0  12.0  12.0  9.0 A1: m = 7, n = 6 9.0 — — — — 9.0 A2: m = 9, n = 6 — — — — — — A3: m = 10, n = 9 — 9.0 — — — A4: m = 16, n = 7 — — 9.0 — — — A5: m = 15, n = 10 — — — 9.0 — — A6: m = 16, n = 15 — — — — 9.0 — C18-MA 9.0 9.0 9.0 9.0 9.0 9.0 C12-MA — — — — — 3.0 CDTBA  0.019  0.021  0.018  0.020  0.018  0.019 AIBN  0.003  0.004  0.003  0.003  0.003  0.004 Synthesis Condition 1-3 1-3 1-3 1-3 1-3 1-3 Yield (%) 93.8  94.8  95.9  93.8  95.7  93.9 

TABLE 6 Example Example Example Example Example Example 1-13 1-14 1-15 1-16 1-17 1-18 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 40 30 A1: m = 7, n = 6 30 — — — — 30 A2: m = 9, n = 6 — — — — — — A3: m = 10, n = 9 — 30 — — — — A4: m = 16, n = 7 — — 30 — — — A5: m = 15, n = 10 — — — 30 — — A6: m = 16, n = 15 — — — — 30 — C18-MA 30 30 30 30 30 30 C12-MA — — — — — 10 Mw 281,000 274,000 276,000 269,000 288,000 270,000 Mn 238,000 230,000 246,000 247,000 244,000 252,000 Mw/Mn 1.18 1.19 1.12 1.09 1.18 1.07 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Balance Performance Additive 9.5 9.5 9.5 9.5 9.5 9.5 Viscosity Index Improver 2.2 2.3 2.5 2.4 2.7 2.5 Kinematic Viscosity 7.22 7.23 7.19 7.29 7.35 7.28 (mm²/s)/100° C. Viscosity Index 190 191 189 192 192 193 HTHS Viscosity (mPa · s) 150° C. 2.6 2.6 2.6 2.6 2.6 2.6 100° C. 4.85 4.75 4.83 4.88 4.91 4.81 MRV Viscosity 12,600 13,500 14,100 14,500 15,300 13,300 (mPa · s)/−40° C.

TABLE 7 Example Example Example Example Example 1-19 1-20 1-21 1-22 1-23 Amount Blended (g) C1-MA 12.0 12.0 12.0 12.0 7.5 A1: m = 7, 9.0 9.0 — — — n = 6 A2: m = 9, — — — — 16.5 n = 6 A3: m = 10, — — — — — n = 9 A4: m = 16, — — — — — n = 7 A5: m = 15, — — — — — n = 10 A6: m = 16, — — 9.0 9.0 — n = 15 C18-MA 9.0 9.0 9.0 9.0 6.0 C12-MA — — — — — CDTBA 0.075 0.031 0.080 0.021 0.022 AIBN 0.011 0.005 0.013 0.004 0.004 Synthesis 1-3 1-3 1-3 1-3 1-2 Condition Yield (%) 98.5 95.6 97.8 95.5 95.2

TABLE 8 Example Example Example Example Example 1-19 1-20 1-21 1-22 1-23 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 25 A1: m = 7, n = 6 30 30 — — — A2: m = 9, n = 6 — — — — 55 A3: m = 10, n = 9 — — — — — A4: m = 16, n = 7 — — — — — A5: m = 15, n = 10 — — — — — A6: m = 16, n = 15 — — 30 30 — C18-MA 30 30 30 30 20 C12-MA — — — — — Mw 120,000 240,000 108,000 270,000 261,000 Mn 107,000 162,000 100,000 185,000 209,000 Mw/Mn 1.12 1.48 1.08 1.46 1.25 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Performance Additive 9.5 9.5 9.5 9.5 9.5 Viscosity Index Improver 2.8 2.3 2.9 2.4 2.5 Kinematic Viscosity 7.44 7.29 7.39 7.51 7.77 (mm²/s)/100° C. Viscosity Index 192 189 191 192 193 HTHS Viscosity (mPa · s) 150° C. 2.6 2.6 2.6 2.6 2.6 100° C. 4.93 4.92 4.91 4.93 5.28 MRV Viscosity 11,800 13,100 14,500 14,800 12,800 (mPa · s)/−40° C.

TABLE 9 Comparative Comparative Comparative Comparative Example 1-1 Example 1-2 Example 1-3 Example 1-4 Amount Blended (g) C1-MA 12.0 12.0 12.0 12.0 A1: m = 7, — — — — n = 6 A2: m = 9, 9.0 9.0 — — n = 6 A3: m = 10, — — — — n = 9 A4: m = 16, — — — — n = 7 A5: m = 15, — — — — n = 10 A6: m = 16, — — 9.0 9.0 n = 15 C18-MA 9.0 9.0 9.0 9.0 C12-MA — — — — CDTBA 0.091 0.028 — 0.088 AIBN 0.016 0.005 0.006 0.015 Synthesis 1-3 1-1 1-4 1-2 Condition Yield (%) 96.2 95.6 94.8 93.9

TABLE 10 Comparative Comparative Comparative Comparative Example 1-1 Example 1-2 Example 1-3 Example 1-4 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 A1: m = 7, n = 6 — — — — A2: m = 9, n = 6 30 30 — — A3: m = 10, n = 9 — — — — A4: m = 16, n = 7 — — — — A5: m = 15, n = 10 — — — — A6: m = 16, n = 15 — — 30 30 C18-MA 30 30 30 30 C12-MA — — — — Mw 88,000 247,000 256,000 95,000 Mn 72,000 152,000 155,000 74,000 Mw/Mn 1.22 1.63 1.65 1.25 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Performance Additive 9.5 9.5 9.5 9.5 Viscosity Index Improver 3.1 2.5 2.6 3.0 Kinematic Viscosity 7.85 7.91 7.88 7.79 (mm²/s)/100° C. Viscosity Index 193 195 193 194 HTHS Viscosity (mPa · s) 150° C. 2.6 2.6 2.6 2.6 100° C. 5.41 5.48 5.39 5.52 MRV Viscosity 13,300 15,100 14,800 14,300 (mPa · s)/−40° C.

TABLE 11 Comparative Comparative Comparative Reference Example 1-5 Example 1-6 Example 1-7 Example 1-1 Amount Blended (g) C1-MA 12.0 12.0 12.0 12.0 A1: m = 7, 9.0 9.0 — — n = 6 A2: m = 9, — — — — n = 6 A3: m = 10, — — — — n = 9 A4: m = 16, — — — — n = 7 A5: m = 15, — — — — n = 10 A6: m = 16, — — — — n = 15 C18-MA 9.0 9.0 9.0 18.0 C12-MA — — 9.0 — CDTBA 0.087 — 0.028 0.091 AIBN 0.016 0.15 0.005 0.016 Synthesis 1-2 1-4 1-1 1-2 Condition Yield (%) 95.1 93.9 95.6 96.2

TABLE 12 Comparative Comparative Comparative Reference Example 1-5 Example 1-6 Example 1-7 Example 1-1 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 A1: m = 7, n = 6 30 30 — — A2: m = 9, n = 6 — — — — A3: m = 10, n = 9 — — — — A4: m = 16, n = 7 — — — — A5: m = 15, n = 10 — — — — A6: m = 16, n = 15 — — — — C18-MA 30 30 30 60 C12-MA — — 30 — Mw 96,000 256,000 247,000 230,000 Mn 76,800 157,000 152,000 188,000 Mw/Mn 1.29 1.63 1.63 1.22 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Performance Additive 9.5 9.5 9.5 9.5 Viscosity Index Improver 3.2 3.2 2.5 3.1 Kinematic Viscosity 7.86 7.99 7.91 7.85 (mm²/s)/100° C. Viscosity Index 92 194 195 193 HTHS Viscosity (mPa · s) 150° C. 2.6 2.6 2.6 2.6 100° C. 5.33 5.55 5.48 4.88 MRV Viscosity 13,900 13,700 15,100 Y.S. (mPa · s)/−40° C.

Example 2-1

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 2-1”).

12 g of methyl methacrylate (compound in which both R¹ and R³ in the formula (6) are methyl groups, and hereinafter, designated as “C1-MA”), 9 g of 2-octyldodecyl methacrylate (compound in which R¹ and R² in the formula (5) are a methyl group and the formula (3) having m=9 and n=6, respectively, and hereinafter, designated as “A2”), 9 g of stearyl methacrylate (compound in which R¹ and R³ in the formula (6) are a methyl group and a stearyl group (straight-chain alkyl group having 18 carbon atoms), respectively, and hereinafter, designated as “C18-MA”), 0.075 g of cumyl dithiobenzoic acid (CDTBA), and 30 g of a highly-refined mineral oil as a solvent were charged into a 300 ml five-neck separable flask fitted with an anchor-type metal stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a sample inlet, and a homogeneous solution was obtained under stirring. The solution was cooled to 0° C. on an ice bath, and vacuum deaeration/nitrogen purge of a reaction system was carried out 5 times using a diaphragm pump. Furthermore, from the sample inlet, as a radical initiator, 0.020 g of azobisisobutyronitrile (AIBN) was charged under nitrogen flow, and then, polymerization was carried out for 12 hours at the solution temperature of 110° C. under a nitrogen atmosphere to obtain a solution containing a poly(meth)acrylate-based viscosity index improver.

For the obtained poly(meth)acrylate-based viscosity index improver, the weight-average molecular weight Mw and the number average molecular weight Mn were measured by GPC analysis. As a result, the weight-average molecular weight Mw was 84000, the number average molecular weight Mn was 53000, and Mw/Mn was 1.58. The procedure of the GPC analysis is as follows.

A solution whose sample concentration is 2 mass % was prepared by dilution using tetrahydrofuran as a solvent. The sample solution was analyzed using GPC equipment (Waters Alliance2695). The analysis was carried out at the flow rate of the solvent of 1 ml/min, by using a column whose analyzable molecular weight is 10000 to 256000, and a refractive index as a detector. It is to be noted that the relationship between the column retention time and the molecular weight was determined using a polystyrene standard whose molecular weight is definite and the molecular weight was determined from the obtained retention time based on the calibration curve which was separately made.

Example 2-2

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 2-2”).

12 g of methyl methacrylate (C1-MA), 9 g of 2-octyldodecyl methacrylate (A2), 9 g of stearyl methacrylate (C18-MA), 0.075 g of cumyl dithiobenzoic acid (CDTBA), and 30 g of a highly-refined mineral oil as a solvent were charged into a 300 ml five-neck separable flask fitted with an anchor-type metal stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a sample inlet, and a homogeneous solution was obtained under stirring. The solution was cooled to 0° C. on an ice bath, and vacuum deaeration/nitrogen purge of a reaction system was carried out 5 times using a diaphragm pump. Furthermore, from the sample inlet, as a radical initiator, 0.018 g of azobisisobutyronitrile (AIBN) was charged under nitrogen flow, and then, polymerization was carried out for 12 hours at the solution temperature of 100° C. under a nitrogen atmosphere to obtain a solution containing a poly(meth)acrylate-based viscosity index improver.

For the obtained poly(meth)acrylate-based viscosity index improver, GPC analysis was carried out in the same manner as Example 2-1, and as a result, the weight-average molecular weight Mw was 77000, the number average molecular weight Mn was 59000, and Mw/Mn was 1.32.

Example 2-3

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 2-3”).

12 g of methyl methacrylate (C1-MA), 9 g of 2-octyldodecyl methacrylate (A2), 9 g of stearyl methacrylate (C18-MA), 0.075 g of cumyl dithiobenzoic acid (CDTBA), and 30 g of a highly-refined mineral oil as a solvent were charged into a 300 ml five-neck separable flask fitted with an anchor-type metal stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a sample inlet, and a homogeneous solution was obtained under stirring. The solution was cooled to 0° C. on an ice bath, and vacuum deaeration/nitrogen purge of a reaction system was carried out 5 times using a diaphragm pump. Furthermore, from the sample inlet, as a radical initiator, 0.014 g of azobisisobutyronitrile (AIBN) was charged under nitrogen flow, and then, polymerization was carried out for 12 hours at the solution temperature of 90° C. under a nitrogen atmosphere to obtain a solution containing a poly(meth)acrylate-based viscosity index improver.

For the obtained poly(meth)acrylate-based viscosity index improver, GPC analysis was carried out in the same manner as Example 2-1, and as a result, the weight-average molecular weight Mw was 88000, the number average molecular weight Mn was 79000, and Mw/Mn was 1.11.

Comparative Example 2-3

A poly(meth)acrylate-based viscosity index improver was synthesized in the following condition (designated as “Synthesis Condition 2-4”).

30 g of a highly-refined mineral oil as a solvent was charged into a 300 ml four-neck reaction flask fitted with a stirring blade (with vacuum seal), a Dimroth condenser, a three-way cock for introducing nitrogen, and a dropping funnel for introducing a sample, and it was stirred for 1 hour in an oil bath at 85° C. while carrying out nitrogen purge. A raw material in which 12 g of methyl methacrylate (C1-MA), 9 g of 2-octyldodecyl methacrylate (A2), and 9 g of stearyl methacrylate (C18-MA) as raw material monomers, and 0.21 g of azobisisobutyronitrile (AIBN) as a radical initiator are mixed was charged into the dropping funnel for introducing a sample, and the raw material was dropped in the reaction flask for 70 minutes. After that, polymerization was carried out for 8 hours at 85° C. under nitrogen flow while maintaining stirring to obtain a solution containing a poly(meth)acrylate-based viscosity index improver. After that, unreacted monomers were removed from the above-described solution by carrying out vacuum distillation for 3 hours at 130° C. and 1 mmHg.

For the obtained poly(meth)acrylate-based viscosity index improver, GPC analysis was carried out in the same manner as Example 2-1, and as a result, the weight-average molecular weight Mw was 98000, the number average molecular weight Mn was 47000, and Mw/Mn was 2.1.

Examples 2-4 to 2-19, Comparative Examples 2-1 to 2-2, 2-4 to 2-7

A poly(meth)acrylate-based viscosity index improver was synthesized in the same manner as any of the above-described Synthesis Conditions 2-1 to 2-4 other than changing the amount of the raw material blended as shown in Tables 13, 15, 17, 19, and 21. It is to be noted that, in Tables, C12-MA represents a compound in which R¹ and R² in the formula (6) are a methyl group and a dodecyl group (straight-chain alkyl group having 12 carbon atoms), respectively, and moreover, A1: m=7, n=6 or the like represents a compound in which R¹ and R² in the formula (5) are a methyl group and the formula (3) having m=7 and n=6, respectively or the like. Mw, Mn, and Mw/Mn of the obtained poly(meth)acrylate-based viscosity index improver are shown in Tables 14, 16, 18, 20, and 22.

<Preparation of Lubricating Oil Composition>

The poly(meth)acrylate-based viscosity index improver obtained in each of Examples 2-1 to 2-19 and Comparative Examples 2-1 to 2-7, performance additives including a metallic (calcium sulfonate whose TBN is 300 mgKOH/g) cleaner, an ashless dispersant (succinimide), a friction modifier (oleylamide), a wear inhibitor (phosphoric acid), an antioxidant (diphenylamine), a metal deactivator (thiadiazole), and a sulfur additive (sulfide ester), and a highly-refined mineral oil (Group II base oil, kinematic viscosity at 100° C.: 3.3 mm²/s, VI: 110) were blended at a ratio shown in Tables 14, 16, 18, 20, and 22 to prepare a lubricating oil composition.

<Evaluation of Lubricating Oil Composition>

For each lubricating oil composition of Examples 2-1 to 2-19 and Comparative Examples 2-1 to 2-7, the kinematic viscosity at 100° C., the viscosity index, and the BF viscosity at −40° C. were respectively measured by methods in conformity with the following. The results are shown in Tables 12, 14, 16, 18, 20, and 22.

kinematic viscosity: JIS K-2283-1993

viscosity index: JIS K 2283-1993

BF viscosity: JPI-5S-26-99

Moreover, the friction property of each lubricating oil composition of Examples 2-1 to 2-19 and Comparative Examples 2-1 to 2-7 was evaluated by a friction coefficient in a condition of constant load using a two cylinder rolling sliding friction tester. Specifically, a friction coefficient was averaged for 10 minutes from the start of the test in conditions where the test temperature is 80° C., the load is 142 N, the surface pressure is 0.48 GPa, the peripheral speed is 1.0 m/s, and the sliding ratio is 5.1%. The results are shown in Tables 14, 16, 18, 20, and 22.

TABLE 13 Example Example Example Example Example Example 2-1 2-2 2-3 2-4 2-5 2-6 Amount Blended (g) C1-MA 12.0  12.0  12.0  12.0  12.0  12.0  A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 9.0 9.0 9.0 9.0 9.0 9.0 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 9.0 9.0 9.0 9.0 9.0 9.0 C12-MA — — — — — — CDTBA  0.075  0.075  0.075  0.402  0.398  0.401 AIBN  0.020  0.018  0.014  0.080  0.075  0.066 Synthesis Condition 2-1 2-2 2-3 2-1 2-2 2-3 Yield (%) 94.1  93.9  94.8  95.3  95.8  94.9 

TABLE 14 Example Example Example Example Example Example 2-1 2-2 2-3 2-4 2-5 2-6 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 40 40 A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 30 30 30 30 30 30 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 30 30 30 30 30 30 C12-MA — — — — — — Mw 84,000 77,000 88,000 17,000 18,000 19,000 Mn 53,000 59,000 79,000 11,000 14,000 17,000 Mw/Mn 1.58 1.32 1.11 1.53 1.31 1.11 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Balance Performance Additive 12.0 12.0 12.0 12.0 12.0 12.0 Viscosity Index Improver 1.8 2.1 2.2 2.9 3.1 3.2 Kinematic Viscosity 5.83 5.85 5.79 5.81 5.79 5.83 (mm²/s)/100° C. Viscosity Index 159 161 162 159 155 156 Friction Coefficient by 0.028 0.027 0.024 0.026 0.025 0.027 Two Cylinder Test BF Viscosity 19,300 19,000 18,400 17,800 17,500 17,000 (mPa · s)/−40° C.

TABLE 15 Example Example Example Example Example Example 2-7 2-8 2-9 2-10 2-11 2-12 Amount Blended (g) C1-MA  6.0 7.5 9.0 13.5  12.0 12.0 A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 12.0 15.0  9.0 9.0  6.0 12.0 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — — — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 12.0 7.5 12.0  7.5 12.0  6.0 C12-MA — — — — — — CDTBA   0.015  0.014  0.016  0.014   0.015   0.016 AIBN   0.026  0.026  0.025  0.025   0.026   0.028 Synthesis Condition 2-3 2-3 2-3 2-3 2-3 2-3 Yield (%) 94.1 93.9  94.8  95.3  95.8 94.9

TABLE 16 Example Example Example Example Example Example 2-7 2-8 2-9 2-10 2-11 2-12 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 20 25 30 45 40 40 A1: m = 7, n = 6 — — — — — — A2: m = 9, n = 6 40 50 30 30 20 40 A3: m = 10, n = 9 — — — — — — A4: m = 16, n = 7 — — ' — — — A5: m = 15, n = 10 — — — — — — A6: m = 16, n = 15 — — — — — — C18-MA 40 25 40 25 40 20 C12-MA — — — — — — Mw 54,000 58,000 51,000 57,000 58,000 53,000 Mn 46,000 51,000 45,000 52,000 51,000 47,000 Mw/Mn 1.17 1.14 1.14 1.09 1.14 1.13 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Balance Performance Additive 12.0 12.0 12.0 12.0 12.0 12.0 Viscosity Index Improver 1.8 2.1 2.2 2.9 3.1 3.2 Kinematic Viscosity 5.85 5.81 5.78 5.83 5.81 5.84 (mm²/s)/100° C. Viscosity Index 158 159 158 159 157 158 Friction Coefficient by 0.027 0.029 0.025 0.027 0.025 0.026 Two Cylinder Test BF Viscosity 18,500 18,200 18,100 18,200 18,500 18,200 (mPa · s)/−40° C.

TABLE 17 Example Example Example Example Example 2-13 2-14 2-15 2-16 2-17 Amount Blended (g) C1-MA 12.0 12.0 12.0 12.0 9.0 A1: m = 7, 9.0 — — — 9.0 n = 6 A2: m = 9, — — — — — n = 6 A3: m = 10, — 9.0 — — — n = 9 A4: m = 16, — — 9.0 — — n = 7 A5: m = 15, — — — 9.0 — n = 10 A6: m = 16, — — — — — n = 15 C18-MA 9.0 9.0 9.0 9.0 9.0 C12-MA — — — — 3.0 CDTBA 0.132 0.135 0.128 0.129 0.132 AIBN 0.022 0.025 0.021 0.022 0.021 Synthesis 2-3 2-3 2-3 2-3 2-3 Condition Yield (%) 94.1 93.9 94.8 95.3 94.9

TABLE 18 Example Example Example Example Example 2-13 2-14 2-15 2-16 2-17 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 30 A1: m = 7, n = 6 30 — — — 30 A2: m = 9, n = 6 — — — — — A3: m = 10, n = 9 — 30 — — — A4: m = 16, n = 7 — — 30 — — A5: m = 15, n = 10 — — — 30 — A6: m = 16, n = 15 — — — — — C18-MA 30 30 30 30 30 C12-MA — — — — 10 Mw 61,000 64,000 76,000 69,000 70,000 Mn 51,500 54,000 68,000 63,000 65,000 Mw/Mn 1.18 1.19 1.12 1.09 1.07 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Performance Additive 12.0 12.0 12.0 12.0 12.0 Viscosity Index Improver 1.8 2.1 2.2 2.9 3.2 Kinematic Viscosity 5.86 5.84 5.87 5.83 5.81 (mm²/s)/100° C. Viscosity Index 158 159 158 158 156 Friction Coefficient by 0.026 0.026 0.027 0.027 0.029 Two Cylinder Test BF Viscosity (mPa · s)/ 19,800 19,600 20,100 20,800 21,800 −40° C.

TABLE 19 Example 2-18 Example 2-19 Amount Blended (g) C1-MA 12.0 12.0 A1: m = 7, n = 6 9.0 9.0 A2: m = 9, n = 6 — — A3: m = 10, n = 9 — — A4: m = 16, n = 7 — — A5: m = 15, n = 10 — — A6: m = 16, n = 15 — — C18-MA 9.0 9.0 C12-MA — — CDTBA 0.066 0.030 AIBN 0.112 0.005 Synthesis Condition 2-3 2-3 Yield (%) 98.5 95.6

TABLE 20 Example 2-18 Example 2-19 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 A1: m = 7, n = 6 30 30 A2: m = 9, n = 6 — — A3: m = 10, n = 9 — — A4: m = 16, n = 7 — — A5: m = 15, n = 10 — — A6: m = 16, n = 15 — — C18-MA 30 30 C12-MA — — Mw 12,000 92,000 Mn 10,700 62,000 Mw/Mn 1.12 1.48 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Performance Additive 12.0 12.0 Viscosity Index Improver 1.8 2.1 Kinematic Viscosity (mm²/s)/100° C. 5.86 5.84 Viscosity Index 158 157 Friction Coefficient by 0.028 0.029 Two Cylinder Test BF Viscosity (mPa · s)/−40° C. 18,800 18,600

TABLE 21 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 2-1 Ex. 2-2 Ex. 2-3 Ex. 2-4 Ex. 2-5 Ex. 2-6 Ex. 2-7 Amount Blended (g) C1-MA 12.0  12.0  12.0  12.0  12.0  12.0 12.0  A1: m = 7, n = 6 — — — 9.0 9.0 — — A2: m = 9, n = 6 9.0 — — — — — — A3: m = 10, n = 9 — — — — — — — A4: m = 16, n = 7 — — — — — — — A5: m = 15, n = 10 — — — — — — — A6: m = 16, n = 15 — 9.0 9.0 9.0 9.0 — — C18-MA 9.0 9.0 9.0 — — 18.0 9.0 C12-MA — — — — — — 9.0 CDTBA  0.135 —  0.129  0.131 —   0.129  0.131 AIBN  0.025  0.021  0.022  0.023  0.021   0.022  0.020 Synthesis Condition 2-1 2-4 2-3 2-1 2-4 2-3 2-3 Yield (%) 95.6  94.8  93.9  95.1  93.9  96.2 95.6 

TABLE 22 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 2-1 Ex. 2-2 Ex. 2-3 Ex. 2-4 Ex. 2-5 Ex. 2-6 Ex. 2-7 Alkyl(meth)acrylate Blending Ratio (mass %) C1-MA 40 40 40 40 40 40 40 A1: m = 7, n = 6 — — — 30 30 — — A2: m = 9, n = 6 30 — — — — — — A3: m = 10, n = 9 — — — — — — — A4: m = 16, n = 7 — — — — — — — A5: m = 15, n = 10 — — — — — — — A6: m = 16, n = 15 — 30 30 30 30 — — C18-MA 30 30 30 — — 60 30 C12-MA — — — — — — 30 Mw 17,000 98,000 109,000 118,000 107,000 60,000 57,000 Mn 10,400 47,000 87,000 91,500 65,000 54,000 50,000 Mw/Mn 1.63 2.1 1.25 1.29 1.65 1.12 1.13 Blending Proportion in Lubricating oil composition (mass %) Base Oil Balance Balance Balance Balance Balance Balance Balance Performance 12.0 12.0 12.0 12.0 12.0 12.0 12.0 Additive Viscosity Index 3.1 2.3 1.9 2.1 2.2 3.8 3.1 Improver Kinematic Viscosity 5.81 5.83 5.83 5.81 5.85 5.85 5.81 (mm²/s)/100° C. Viscosity Index 151 150 154 155 152 157 151 Friction Coefficient by 0.043 0.038 0.042 0.043 0.029 0.049 0.043 Two Cylinder Test BF Viscosity 26,600 28,100 23,800 24,200 21,800 67,800 53,600 (mPa · s)/−40° C. 

1. A poly(meth)acrylate-based viscosity index improver comprising a polymer chain comprising a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), wherein a weight-average molecular weight Mw is 100000 or more, and a ratio of the weight-average molecular weight Mw to a number average molecular weight Mn, Mw/Mn, is 1.6 or less.

[In the formulas (1) and (2), R¹ represents hydrogen or a methyl group, and R² represents a group represented by the following formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

In the formulas (3), m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]
 2. A poly(meth)acrylate-based viscosity index improver comprising a polymer chain comprising a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), wherein a weight-average molecular weight Mw is less than 100000, and a ratio of the weight-average molecular weight Mw to a number average molecular weight Mn, Mw/Mn, is 1.6 or less.

[In the formulas (1) and (2), R¹ represents hydrogen or a methyl group, and R² represents a group represented by the following formula (3), and R³ represents a C1 to C18 alkyl group that is straight-chain or has a branch having 5 or less carbon atoms.

In the formula (3), m and n are integers which satisfy m≧5, n≧4, and m+n≦31.]
 3. A lubricating oil additive comprising the poly(meth)acrylate-based viscosity index improver according to claim
 1. 4. A lubricating oil composition comprising: a lubricating base oil; and the poly(meth)acrylate-based viscosity index improver according to claim
 1. 5. A lubricating oil additive comprising the poly(meth)acrylate-based viscosity index improver according to claim
 2. 6. A lubricating oil composition comprising: a lubricating base oil; and the poly(meth)acrylate-based viscosity index improver according to claim
 2. 